Synchronized interference suppression in frequency domain

ABSTRACT

An apparatus for communication includes a transmission chain, an interference suppressor and a reception chain. The transmission chain is configured to transmit a transmitted signal. The interference suppressor is configured to produce suppression signals, by filtering reference signals drawn from the transmission chain. At least one of the reference signals is filtered in a frequency domain. The reception chain is configured to receive a signal distorted by at least an attenuated and delayed replica of the transmitted signal. The transmitted signal and the received signal are (i) synchronized in time and (ii) each includes one or more subcarriers selected from a common subcarrier-resource. The reception chain is further configured to process the received signal up to selected points, to subtract the suppression signals from the received signal at the selected points to produce an interference-suppressed signal, and to recover data carried in the received signal from the interference-suppressed signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication 62/465,193, filed Mar. 1, 2017, whose disclosure isincorporated herein by reference.

TECHNICAL FIELD

Embodiments described herein relate generally to communication systems,and particularly to methods and systems for efficient interferencesuppression.

BACKGROUND

In various wired and wireless communication systems the performance of areceiver is degraded by interfering signals originating from atransmitter. Methods for suppressing such interference are known in theart. For example, U.S. Pat. No. 7,672,447 describes an echo cancellerthat reduces an echo signal produced when the transmitted signal leaksback into the receiver via a hybrid. The echo canceller estimates theecho signal from the transmitted signal, and then the estimated echosignal is subtracted from the received signal. In practice, the echopath channel in a DMT-modem is much longer than the cyclic prefix, andtherefore, the received echo signal will be subjected to both ISI(inter-symbol-interference) and ICI (inter-carrier-interference). Atraditional echo canceller, designed for a xDSL-modem, uses either atime domain adaptive FIR-filter or a combined echo canceller implementedin both time and frequency domain. A matrix-based adaptive echocanceller is implemented in the frequency domain.

U.S. Patent Application Publication 2017/0237492 describes achievingfull duplex bidirectional transmission across coaxial cable in a hybridfiber-coaxial cable TV network. Some preferred systems and methods willattenuate reflections propagated within the coaxial cable. Otherpreferred systems may echo-cancel reflections propagated within thecoaxial cable.

U.S. Patent Application Publication 2017/0019241 describes acommunication system comprising a media access control (MAC) schedulerin a cable network, and a full band transceiver. The MAC schedulerimplements a two-dimensional transmission-reception (T-R) coordinationscheme among a plurality of cable modems in the cable network. The cablemodems are categorized into interference groups for facilitating fullduplex communication in the cable network across the frequency range.The full band transceiver implements an adaptive interferencecancellation scheme, which suppresses at a receiver a signal transmittedby a transmitter.

An international telecommunications standard, referred to as “Data OverCable Service Interface Specification” (DOCSIS) that was developed byCableLabs and contributing companies, specifies the transfer ofhigh-bandwidth data over existing cable TV (CATV) systems. In 2017, aFull Duplex (FDX) version of DOCSIS version 3.1 has been announced, inwhich at least part of the spectrum of the cable plant can be usedsimultaneously in both upstream and downstream directions. The physicallayer of the DOCSIS 3.1 standard, including FDX, is specified, forexample, in Annex F of the specifications “Data-Over-Cable ServiceInterface Specifications DOCSIS® 3.1, Physical Layer Specification,”CM-SP-PHYv3.1-I13-171220, December 2017.

SUMMARY

An embodiment that is described herein provides an apparatus forcommunication that includes a transmission chain, an interferencesuppressor and a reception chain. The transmission chain is configuredto transmit a transmitted signal. The interference suppressor isconfigured to produce one or more suppression signals, by filtering oneor more respective reference signals drawn from the transmission chain.At least a given reference signal of the one or more reference signalsis filtered in a frequency domain. The reception chain configured toreceive a received signal distorted by an interference signal includingat least an attenuated and delayed replica of the transmitted signal.The transmitted signal and the received signal are (i) synchronized intime and (ii) each includes one or more subcarriers selected from acommon subcarrier-resource. The processing chain is further configuredto process the received signal up to one or more selected points alongthe reception chain, to respectively subtract the one or moresuppression signals from the received signal at the selected points toproduce an interference-suppressed signal and to recover data carried inthe received signal from the interference-suppressed signal.

In some embodiments, the transmission chain includes a digital TXsub-chain followed by an analog TX sub-chain, and each of the one ormore reference signals is drawn from the digital TX sub-chain or fromthe analog TX sub-chain. In other embodiments, the reception chainincludes an analog RX sub-chain followed by a digital RX sub-chain, andeach of the selected points along the reception chain is included in theanalog RX sub-chain or in the digital RX sub-chain. In yet otherembodiments, the transmission chain and the reception chain areconfigured to operate in a full duplex mode.

In an embodiment, the interference suppressor is configured to generatean analog suppression signal based on a digital reference signal drawnfrom a digital sub-chain of the transmission chain. In anotherembodiment, the interference suppressor is configured to generate, usingtime-domain filtering, an analog suppression signal for suppressing atleast noise or distortion interference, and to generate, usingfrequency-domain filtering, a digital suppression signal for suppressinginterference caused by leakage or reflections of the transmitted signal.In yet another embodiment, the transmission chain is configured to splitthe transmitted signal into multiple signal-copies for transmission overmultiple respective ports, the reception chain is configured to combinemultiple reception signals received from the respective ports, and theinterference suppressor is configured to generate a suppression signalbased on respective samples of the multiple signal-copies.

In some embodiments, the interference suppressor is configured togenerate for the multiple signal-copies multiple respective temporarysuppression signals, and to generate the suppression signal by summingthe multiple temporary suppression signals. In other embodiments, theinterference suppressor is configured to generate the temporarysuppression signals by applying to each of the signal-copies atime-domain or a frequency-domain filtering operation. In yet otherembodiments, the interference suppressor is configured to generate thesuppression signal for reducing a level of at least noise or distortionpresent at the multiple signal-copies.

In an embodiment, the interference suppressor is configured to subtracta first suppression signal from the received signal at a first point ofthe reception chain, and to subtract a second suppression signal at asecond different point of the reception chain for reducing a residualinterference signal present in the received signal after subtracting thefirst suppression signal at the first point. In another embodiment, thetransmission chain and the reception chain are configured to operate inaccordance with a DOCSIS full duplex specification. In yet anotherembodiment, the transmission chain is configured to transmit thetransmitted signal over a coaxial-based network of a Cable TV (CATV)system, and the reception chain is configured to receive the receivedsignal over the coaxial-based network of a Cable TV (CATV) system.

There is additionally provided, in accordance with an embodiment that isdescribed herein, a method for communication including transmitting atransmitted signal by a transmission chain. One or more suppressionsignals are produced, by filtering one or more reference signals drawnfrom the transmission chain. At least a given reference signal of theone or more reference signals is filtered in a frequency domain. Areceived signal is received in a reception chain, the received signal isdistorted by an interference signal including at least an attenuated anddelayed replica of the transmitted signal. The transmitted signal andthe received signal are (i) synchronized in time and (ii) each includesone or more subcarriers selected from a common subcarrier-resource. Thereceived signal is processed up to one or more selected points along thereception chain, and the one or more suppression signals are subtractedfrom the received signal at the respective selected points to produce aninterference-suppressed signal. Data carried in the received signal isrecovered from the interference-suppressed signal.

These and other embodiments will be more fully understood from thefollowing detailed description of the embodiments thereof, takentogether with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that schematically illustrates a transceiveroperating in a communication system, in accordance with an embodimentthat is described herein;

FIG. 2 is a diagram that schematically illustrates timingsynchronization between transceiver TX chain and RX chain, in accordancewith an embodiment that is described herein;

FIG. 3 is a block diagram that schematically illustrates a multi-legnode that includes interference suppression, in accordance with anembodiment that is described herein; and

FIGS. 4A-4C are diagrams that schematically illustrate channel responsefunctions used for echo suppression, in accordance with an embodimentthat is described herein.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

Embodiments that are described herein provide improved methods andsystems for interference suppression in a communication system.

A transceiver is a communication apparatus comprising a transmitter anda receiver. For example, in a cable TV headend, a transceiver maycommunicate bi-directionally with end users equipped with Cable Modems(CMs) over a wire communication network.

Since the signal transmitted by the transmitter is much stronger thanthe signal received at the receiver, a portion of the transmitted signaltypically leaks to the receiver, e.g., due to imperfect isolation. Thetransmitted signal may also interfere with the received signal, by beingreflected back to the receiver from an antenna or a wire connector atthe transmitter output, or by various elements of the communicationnetwork. Other interference sources include thermal noise, quantizationnoise due to conversion between analog and digital signals, andnonlinear distortion caused mainly by power amplifiers.

Signal reflections are typically caused due to imperfect impedancematching at the point of reflection. In a wireless network, reflectionsmay also be caused by a multipath channel. In the present context, theterm “reflection” refers to an attenuated and delayed replica of thetransmitted signal. The term “reflection” is also referred to as “echo.”

The interfering signal is typically weaker than the originatingtransmitted signal, but its level is sufficiently high, compared to thereceived signal, to degrade the receiver performance.

One approach to avoid transmitter-to-receiver interference is to use aTime Division Duplexing (TDD) or Frequency Division Duplexing (FDD)scheme, i.e., separate between transmission and reception in time orfrequency, respectively. This approach however fails to fully exploittime/frequency resources of the bi-directional communication channel.

The FDX DOCSIS 3.1 standard cited above specifies a full-duplex modeover part of the cable-plant spectrum (also referred to as the FDXBand.) In some disclosed embodiments, for achieving acceptable receiverperformance in the full duplex mode, interference caused to the receiveris canceled or suppressed, as will be described in detail below.

In some embodiments, the transmitter comprises a transmission chain thatprocesses the transmitted signal, and the receiver comprises a receptionchain that processes the received signal. The transceiver additionallycomprises an interference suppressor that emulates an interferencechannel between a TX reference point along the transmission chain and aRX reference point within the reception chain, at which the suppressionsignal is subtracted to produce an interference-suppressed signal. Thereceiver recovers data carried in the received signal from theinterference-suppressed signal. The interference suppressor generatesthe suppression signal by filtering the reference signal in the timedomain, in the frequency domain or both, as will be described below.

In some embodiments, for efficient frequency-domain implementation ofinterference suppression in an Orthogonal Frequency-DivisionMultiplexing (OFDM) signals transceiver, the transmitted OFDM signal andthe received OFDM signal are (i) synchronized in time and (ii) eachcomprises one or more subcarriers selected from a commonsubcarrier-resource.

The TX (or RX) reference point can be any suitable point selected in adigital sub-chain or in an analog sub-chain of the transmission (orreception) chain. In some embodiments, the interference suppressorgenerates two or more suppression signals based on two or morerespective reference signals, and each of the suppression signals issubtracted at a respective RX reference point. In one embodiment, theinterference suppressor filters the reference signal in the time domainusing a convolution operation. In another embodiment, the impulseresponse of the interference channel is long, and the interferencesuppressor efficiently filters the reference signal in the frequencydomain, as will be described below.

In an embodiment, the interference suppressor applies frequency-domainfiltering by converting the reference signal to the frequency domainusing Fast Fourier Transform (FFT), multiplying the frequency domainsignal by a frequency response function of the interference channel, andconverting the result back to the time domain using Inverse FFT (IFFT).In practical applications, frequency-domain filtering may be on theorder of thirty times more efficient than time-domain filtering.Depending on the selection of the TX and RX reference points, one of theFFT and IFFT operations, or both, can be eliminated to further reducethe computational load.

In some embodiments, the transceiver synchronizes between thetransmitter and receiver so that the FFT block at the receiver containsechoes corresponding to only a single transmitted OFDM symbol. In suchembodiments, frequency-domain filtering can be applied efficiently withno overlapping between successive FFT blocks.

In some embodiments, the transmission chain transmits multiplesignal-copies of the transmitted signal over multiple respective ports(or legs). In such embodiments, a different echo may be received fromeach leg. Thus, the reception chain combines multiple reception signalsreceived via the respective ports at the front-end of the receptionchain. To suppress echoes related to the transmitted signal, theinterference suppressor generates a single suppression signal based onthe sum of the reflections of all or some of the multiple signal-copies.To suppress noise and distortion interference, the interferencesuppressor generates for the multiple signal-copies multiple respectivetemporary suppression signals, and produces the suppression signal bysumming the multiple temporary suppression signals. The interferencesuppressor may generate the temporary suppression signals usingtime-domain or frequency-domain filtering.

In some embodiments, the interference suppressor generates thesuppression signal for reducing a level of at least noise and distortionpresent at the multiple signal-copies. The interference suppressorsubtracts the suppression signal from the received signal at a firstpoint of the reception chain, and subtracts another suppression signalat a second different point of the reception chain for reducing aresidual interference signal present in the received signal aftersubtracting the suppression signal at the first point.

In the disclosed embodiments, interference suppression techniques areapplied to improve the receiver performance when operating in a fullduplex mode. In an efficient implementation, the suppression of echointerference is carried out using efficient frequency-domain filteringand the suppression of noise and distortion interference is carried outusing time-domain filtering. The disclosed techniques result in reducedcomputational load, and reduced power consumption.

System Description

FIG. 1 is a block diagram that schematically illustrates a transceiver24 operating in a communication system, in accordance with an embodimentthat is described herein. In the present example, transceiver 24operates in a cable TV (CATV) communication system, e.g., in a headendor node that distributes content to multiple subscribers and/or providesbidirectional access to the Internet. Alternatively, transceiver 24 mayoperate in any other suitable communication system such as a wirelesscommunication system. Further alternatively, transceiver 24 may beimplemented at the end device side, e.g., in a cable modem or in awireless mobile device.

Transceiver 24 comprises a transmitter 28 and a receiver 32 that areboth coupled to a coupling device 36. The coupling device receivesdownstream signals from the transmitter and transmits them to adistribution network 40. Coupling device 36 additionally receivesupstream signals from the distribution network and forwards the upstreamsignals to the receiver.

In the example of FIG. 1, distribution network 40 comprises awired-based distribution network that allows bidirectional communicationbetween transceiver 24 and remote subscriber devices such as CableModems (CMs) 44. Alternatively or additionally, any other suitablesubscriber devices such as set top boxes (not shown) that are compliantwith the DOCSIS specifications can also be used, e.g., in InternetProtocol TV (IPTV) applications. In the present example, distributionnetwork 40 is a wired network comprising taps 46 interconnected usingcoax cables. In the present example, each tap 46 connects to fourrespective CMs via respective drop cables 48.

A tap receiving a downstream signal from transceiver 24, forwards aportion of the downstream signal to each of its interconnected CMs viarespective drop cables 48, and possibly to a successive tap. A tapreceiving an upstream signal from a CM via a drop cable, or from anothertap, forwards the upstream signal to the transceiver possibly via one ormore additional taps.

In alternative embodiments, distribution network 40 may comprise anysuitable communication network such as a wireless network or acombination of a wireless network and a wired-based network.

In the example of FIG. 1, transceiver 24 transmits signals modulatedusing an Orthogonal Frequency-Division Multiplexing (OFDM) scheme, andreceives signals modulated using an Orthogonal Frequency-DivisionMultiple Access (OFDMA) scheme. For example, transceiver 24 maycommunicate OFDM signals in accordance with the FDX DOCSIS 3.1 standardcited above. Alternatively, any other suitable modulation technique canalso be used.

Transmitter 28 processes signals for transmission using a transmissionchain (also denoted TX chain) that comprises several elements asdescribed herein. A QAM modulator 50 receives a stream of input bits(not shown) and maps the input bits into QAM symbols, in accordance witha predefined QAM constellation that comprises up to several thousands ofconstellation points (e.g., 16K-QAM). Alternatively, any other suitablemapping of bits to modulation symbols, e.g., BPSK modulation, can alsobe used. An Inverse Fast Fourier Transform (IFFT) module 52 transformsthe QAM symbols into an OFDM signal that modulates multiple subcarriers.In accordance with the FDX DOCSIS 3.1 standard cited above, the nominalsample rate at the IFFT output is 204.8 MHz, and the IFFT block size maybe 4096 or 8192 samples, corresponding to subcarrier spacing of 50 kHZand 25 KHz, respectively. The downstream bandwidth may be limited to 192MHz. The OFDM signal resulting by applying one operation of the IFFTmodule corresponds to one OFDM symbol.

The transmitter applies to the OFDM signal at the output of IFFT moduleany suitable shaping window such as a raised cosine window (not shown).In some embodiments, the transmitter adds to the OFDM symbol a CyclicPrefix (CP) extension, i.e., a copy of the last Ncp points of the OFDMsymbol at the beginning of the same OFDM symbol (not shown). Theaddition of the CP enables the receiver at the CM side to overcome InterSymbol Interference (ISI).

A downstream (DS) channelizing module 54 receives the OFDM signal afterthe addition of the CP and windowing, and up-converts the OFDM signalfrom baseband to a selected channel within the FDX bandwidth.

A Digital to Analog Converter (DAC) 56 converts the OFDM signal into ananalog form. In some embodiments, the TX chain comprises an up-tiltequalizer 58 that emphasizes high frequencies of the downstream signalto achieve an approximately flat frequency response over the usedbandwidth at the CMs' inputs. The up-tilted downstream signal isamplified by a Power Amplifier (PA) and transmitted to the distributionvia coupling device 36.

In the reception direction, an OFDMA upstream signal generated by a CM44 is received via coupling device 36, amplified by a Low NoiseAmplifier (LNA) 68, filtered by a receiver filter 70 and sampled andconverted into a digital form using an Analog to Digital Converter (ADC)72. Receiver filter 70 may comprise any suitable low-pass or bandpassfilter, e.g., designed as a pre sampling filter. The upstream signal isthen processed by an upstream (US) channelizing module 74, whichdown-converts the upstream signal to baseband. The receiver may apply tothe down-converted signal a suitable shaping window, e.g., for reducinginter-carrier interference. The shaping window may be the same as ordifferent from the shaping window applied in the TX chain. The output ofUS channelizing module 74 comprises OFDM symbols. The receiver removesthe CP from the OFDMA symbols, and a Fast Fourier Transform (FFT) module76 converts the OFDMA symbols into respective QAM symbols in one or morepredefined constellations. A QAM demodulator 78 maps the QAM symbolsinto bits in accordance with the underlying constellations.Alternatively, other suitable constellations such as BPSK can also beused.

In the disclosed embodiments, transceiver 24 generally operates in afull duplex mode, i.e., one or more frequency subcarriers may be usedsimultaneously by both the transmitter and receiver. As such, thedownstream signals transmitted by the transmitter may interfere with theupstream signals received by the receiver. To mitigate suchinterference, transceiver 24 comprises an analog interference suppressor82, a digital interference suppressor 84, or both. During any timeperiod, the transceiver may activate only analog interference suppressor82, only digital interference suppressor 84, or both.

Analog interference suppressor 82 generates an analog suppressionsignal. The analog suppression signal can be subtracted from thereceived signal at any suitable RX reference point of the RX chainbetween the output of coupling device 36 and the input to ADC 72. In thepresent example, the receiver subtracts the analog suppression signalfrom the received signal at the output of coupling device 36, orequivalently the input of LNA 68, thus preventing LNA 68, ADC 72 or bothfrom being saturated by the interfering signal. In the present example,the reference signal input to the analog interference suppressor isdrawn from an element of the digital part of TX chain 28 such as QAMmodulator 50, IFFT module 52 or DS channelizing module 54. A DAC 88converts the reference signal to an analog form as input for analoginterference suppressor 82. In alternative embodiments, the referencesignal can be drawn from the analog part of TX chain 28, e.g., from theoutput of DAC 56, up-tilt equalizer 58 or PA 60, in which case DAC 88 isomitted.

Digital interference suppressor 84 receives a reference signal from aselected reference point within the TX chain. The reference signal maybe drawn from a digital sub-chain of the TX chain comprising QAMmodulator 50, IFFT module 52 and the DS channelizing module 54.Alternatively, an analog reference signal may be drawn from the analogsub-chain of the TX chain comprising DAC 56, up-tilt equalizer 58 and PA60. The goal of digital interference suppressor 84 is to reduce thelevel of interference when analog interference suppressor 82 is omittedfrom the implementation or disabled, or to reduce the residualinterference remaining after applying the analog interferencesuppressor, so as to meet a Signal-to-Interference-plus-Noise Ratio(SINR) requirement at the receiver.

The suppression signal generated by the digital interference suppressorcan by subtracted from the received signal at any suitable RX referencepoint along the digital part of the RX chain after ADC 72. In thepresent example, the RX reference point can be the input to USchannelizing module 74, FFT module 76 or QAM demodulator 78.

Digital interference suppressor 84 comprises an interference-emulatingfilter 86 that emulates the underlying interference channel. Dependingon the selected TX reference point in the TX chain and on the RXreference point selected in the RX chain, digital interferencesuppressor 84 may additionally comprise one or more processing elementssimilar to respective elements of the TX or RX chain.

Example routes from selected TX reference points to RX reference pointsvia the digital interference suppressor are depicted as bold lines inthe figure. For example, when the TX reference signal is drawn from theoutput of PA 60 and the RX reference point is the input to FFT module76, the digital interference suppressor comprises elements emulating achannel comprising LNA 68, receiver filter 70, ADC 72, US channelizingmodule 74, as well as coupling device 36 and distribution network 40channels. As another example, when the TX reference signal is drawn fromthe output of DS channelizing module 54, and the RX reference point isthe input to FFT module 76, the digital interference suppressorcomprises channelizing module similar to US channelizing module 74 (notshown) and interference-emulating filter 86. The channelizing module ofthe digital interference suppressor receives samples of the fullspectrum as input and reduces each of the US channels to baseband (theDOCSIS FDX specifications support up to six US channels). Theinterference-emulating filter convolves each of the baseband channelswith the respective echo channel response. This echo channel emulatesthe physical channel between the TX reference point and the subtractionpoint. In this example, the echo channel response emulates the elements:DAC 56, up-tilt equalizer 58, PA 60, coupling device 36, relevant partsof distribution network 40, LNA 68, receiver filter 70, and USchannelizing module 74.

As yet another example, when the TX reference signal is drawn from theoutput of QAM modulator 50, and the RX reference point is the input toQAM demodulator 78, no processing elements other than frequency-domaininterference-emulating filter 86 are required.

Embodiments Implementing the Interference-Emulating Filter

In some embodiments, interference-emulating filter 86 comprises adigital Finite Impulse Response (FIR) filter whose time-domain (TD)coefficients emulate the impulse response function of the interferencechannel. In other embodiments, the interference-emulating filter 86 isrepresented by frequency-domain (FD) coefficients that emulate thefrequency response function of the interference channel.Interference-emulating filter 86 emulates the interference channelbetween the TX reference point of the TX chain and the RX referencepoint for subtracting the estimated interference signal in the RX chain.

In the context of the present application and in the claims, the term“time-domain filtering” refers to a filtering operation that is based ona convolution operation with an impulse response function of theunderlying interference channel. More generally, “time-domain filtering”refers to processing of time-domain samples of the TX reference signaland using time-domain representation of the interference channel,without transforming any of the TX reference signal and the interferencechannel representation to the frequency domain. Time-domain filteringcan be implemented, for example, using a Finite Impulse Response (FIR)filter in which case the channel representation comprises thetime-domain impulse response of the interference channel. Alternatively,time-domain filtering can be implemented using an Infinite ImpulseResponse (IIR) filter in which case the interference channel isrepresented using feed-forward and feed-backward coefficients. The term“frequency-domain filtering” refers to a filtering operation based onmultiplication of a frequency domain representation of the TX referencesignal by a frequency response function of the underlying interferencechannel.

Digital interference suppressor 84 can estimate the impulse responsefunction or the frequency response function of the interference channel(i.e., the TD or FD coefficients of the filter) using any suitablemethod, e.g., data-aided estimation methods such as, for example, LeastMean Squares (LMS), Minimum Mean Square Error (MMSE), Recursive LeastSquares (RLS) or Maximum Likelihood (ML) estimation. Estimating theinterference channel can be carried out in parallel to receivingupstream signals, or during periods in which no upstream signals aretransmitted by any of the remote CMs. In some embodiments digitalinterference suppressor 84 tracks time variations of the interferencechannel by re-estimating the interference channel periodically.

In an embodiment, interference-emulating filter 86 generates aninterference suppression signal by applying a convolution operationbetween the estimated TD coefficients and samples of the TX referencesignal, or a processed version of the TX reference signal by one or morechain elements as described above. The computational complexity offiltering using a convolution operation depends on the number of TDcoefficients N_(TD) required for representing the interference channelreliably, and on the sampling rate Fs used in the RX chain. In terms ofComplex Multiply-and-Accumulate (CMAC) operations per second, thecomplexity is given by (N_(TD)·Fs) CMAC operations per second.

Consider, for example, an interference channel whose impulse response is4 microseconds long. At a sampling rate of 100 MHz, the impulse responsefunction of the interference channel is represented by N_(TD)=400coefficients and the complexity of the digital interference suppressoris 40 G CMACs/second.

In some embodiments, to reduce complexity, interference-emulating filter86 applies filtering in the frequency domain (FD). In such embodiments,interference-emulating filter 86 is implemented using the FDcoefficients representing the frequency response function of theinterference channel, at the respective frequencies of the IFFT output.The interference-emulating filter converts the TX reference signal tothe frequency domain using a FFT operation to produce a FD referencesignal and multiplies the FD reference signal by the FD coefficients(i.e., an element-by-element multiplication) to produce a FDinterference suppression signal.

In some embodiments, the signal at the TX reference point is given inthe frequency domain (the input to IFFT module 52) and the FDinterference suppression signal is subtracted in the frequency domain atthe output of FFT module 76.

Let N_(FFT) denote the FFT size used. The complexity of a single FFT (orIFFT) operation (assuming a radix-2 implementation) is given byC_(FFT)=[(N_(FFT)/2)·log₂(N_(FFT))] CMAC operations. The FD filteringcomplexity assuming subtraction in the frequency domain is thus given by(Fs/N_(FFT))·[N_(FFT)+2·C_(FFT)]. For a 4K-FFT at a 100 MHz sample rate,the resulting complexity is (1e8/4096)·[4096+4096.12], or 1.3 GCMACs/second. In some embodiments, FD filtering requires someblock-overlapping, in which case the complexity is slightly higher than1.3 G CMACs/second. This computation is significantly more efficientthan the 40 G CMAC operations per second using a convolution operationas described above.

In some embodiments, the TX chain transmits OFDM downstream signals andthe RX chain receives OFDMA upstream signals. In such embodiments, anequivalent FD filtering operation, in which only the frequency domainelement-by-element multiplication is required by interference-emulatingfilter 86, can be applied by using the output of QAM modulator 50 as aTX reference point and the input to QAM demodulator 78 as a RX referencepoint, instead of taking (for example) the output of IFFT module 52 as aTX reference point and the input of FFT module 76 as the RX referencepoint, and filtering the TX reference signal using a convolutionoperation. This embodiment makes use of the IFFT and FFT operations thatare already placed in the TX and RX chains for implementing theunderlying OFDM scheme. The computational complexity in this embodiment(given Fs=100 MHz) is only 0.1 G CMAC operations per second.

In some embodiments, a Cyclic Prefix (CP) is inserted per OFDM symbol.In such embodiments, implementing the FD filtering requires nooverlapping between successive FFT blocks. Moreover, by skipping the CPintervals the computational complexity can be further reduced.

In some embodiments, the TX reference signal is drawn from the output ofPA 60. In such embodiments, the interference channel includes noise andnonlinear distortion that appear at the PA output and cannot becompensated for when the TX reference point resides prior to PA 60.Assuming that the RX reference point is selected at the output of theFFT module, only one FFT operation to the sampled and digitized outputof the PA is required. The computational complexity in this case isgiven by (Fs/N_(FFT))·[N_(FFT)+C_(FFT)]. Using the parameters aboveresults in (1e8/4096)·[4096+2048.12]=0.7 G CMACs/second. Table 1 belowsummarizes several implementation options and related complexity. Notethat in taking the CP into consideration, the complexity numbersdepicted in Table 1 would be slightly lower.

TABLE 1 Interference-emulating filter implementation options andcomputational complexity Domain Implementation option CMACs/second TimeConvolution  40G Frequency FFT + IFFT + Multiplication 1.3G FrequencyFFT + Multiplication 0.7G Frequency Multiplication only 0.1G

For efficient implementation of frequency-domain filtering in atransceiver that processes OFDM signals in both the downstream (TX) andupstream (RX) directions, the following four requirements apply, ofwhich first three are related to a time-synchronization requirementbetween the receiver and transmitter:

-   -   1. The OFDM symbol time is the same for both the TX and RX        directions. Since in full duplex the same bandwidth is shared        for transmission and reception, an equivalent requirement is        that the same OFDM subcarrier spacing should be used for both TX        and RX directions.    -   2. The CP length is the same for both TX and RX directions.    -   3. The symbol timing in the TX chain and in the RX chain are        inter-synchronized so that the interval over which the FFT        module in the RX chain applies the FFT operation contains        reflections originating from a single OFDM symbol transmitted by        the TX chain.    -   4. The OFDM subcarriers in both the TX and RX directions share a        common set of subcarriers on a predefined frequency grid. This        grid of subcarriers is a predefined set of center frequencies        from which the subcarrier frequencies of the US and DS are        selected, also referred to in the present context and in the        claims as a “common subcarrier-resource”. This requirement        ensures orthogonality between the subcarriers used for the US        and DS directions.

FIG. 2 is a diagram that schematically illustrates timingsynchronization between TX chain and RX chain of a transceiver, inaccordance with an embodiment that is described herein. The timingscheme is described with reference to transceiver 24 of FIG. 1, but isrelevant also to transceiver 100 of FIG. 3 that will be described below.

In the example of FIG. 2, SYM1, SYM2 and SYM3 denote OFDM symbolstransmitted by transmitter 28 of transceiver 24, whereas SYM4, SYM5 andSYM 6 denote OFDM symbols received by receiver 32 of transceiver 24.Each OFDM symbol includes a respective CP.

In the present example, the transmitted signal as well as echo signalsof the transmitted signal denoted ECHO1 and ECHO2, reflected from thedistribution network are received by the receiver as interference. Inaddition, the received signal as well as echoes ECHO3 and ECHO4 of thereceived signal are also present at the receiver input.

AS seen in the figure, the same symbol length and CP length are used forboth TX and RX directions. In addition, the IFFT operation (IFFT block)in the TX chain and the FFT operation (FFT block) in the RX chain aretime-synchronized so that echoes of only one transmitted OFDM symbolappear within each FFT block. For example, only echoes of SYM1 and CP1fall within the FFT block applied to the received signal CP4 and SYM4.Consequently, circular convolution resulting by applying FD filtering isequivalent to a linear convolution in the time domain.

Multi-Leg Node with Interference Suppression

FIG. 3 is a block diagram that schematically illustrates a multi-legnode 100 that includes interference suppression, in accordance with anembodiment that is described herein. Multi-leg node 100 comprises atransceiver that comprises a TX chain 104 and a RX chain 108. Amulti-leg configuration enables efficient transmission of the same datato multiple subscribers with minimal hardware duplication.

FIG. 3 depicts only the analog part of the TX chain starting with a DAC120 that converts a downstream signal from a digital form to an analogform. An up-tilt equalizer 124 emphasizes high frequencies of thedownstream signal to compensate for the channel response, and a splitter128 that splits the downstream signal into four signal-copies that areamplified separately by four respective Power Amplifiers (PAs) 132. Inalternative embodiments, splitter 128 splits the transmitted signal intoany suitable number of signal-copies other than four.

The outputs of PAs 132 are sampled using respective TX couplers 136 andtransmitted to the distribution network via respective RX couplers 148and respective output legs denoted LEG1 . . . LEG4. Each of the legstypically comprises a Radio Frequency (RF) connector.

In the present context we assume that CMs 44 connected to LEG1 . . .LEG4 belong to a common DOCSIS service group as specified, for example,in the FDX DOCSIS 3.1 standard cited above. The legs serving a givenservice group transmit the same downstream signal, and receiverespective upstream signals assigned to different time/frequencyresources, i.e., mini-slots containing multiple subcarriers.

The above assumption typically does not hold for mini-slots assigned tocollision-based messages such as request or initial ranging. Collisionsmay also occur on such mini-slots by CMs coupled to the same leg. Notethat even though there are several physical legs, the underlyingscheduling scheme assumes that time-frequency resources are sharedbetween all CMs as if they were coupled to the same leg.

The common service group assumption is needed for combining the legs inthe Rx path without the risk of a collision, i.e., transmission of CMsfrom different legs on the same mini-slots is not allowed (excludingtransmissions that are collision-based by nature as described above).

Rx chain 108 receives upstream signals from LEG1 . . . LEG4 viarespective RX couplers 148. The upstream signals are filtered usingrespective Low Pass Filters 152, amplified using respective Low NoiseAmplifiers (LNAs) 156, and combined using a four-to-one combiner 162. Inan alternative embodiment, the outputs of RX couplers 148 are firstcombined using four-to-one combiner 162, and then filtered and amplifiedusing LPF 152 and LNA 156 or other suitable filter and amplifier. Thecombined upstream signal output by combiner 162 is sampled and convertedby an ADC 164 into a digital upstream signal.

Rx chain 108 further comprises an upstream (US) channelizing module 168,a FFT module 172 and a QAM demodulator 174 that sequentially process thedigital upstream signal to recover data sent from the CMs, similarly toUS channelizing module 74, FFT module 76 and a QAM demodulator 78 ofFIG. 1.

Multi-leg node 100 comprises a multi-ref interference suppressor 180whose TX reference points are the outputs of PA 132, and a single-refinterference suppressor 190 whose TX reference point is drawn from theTX chain prior to DAC 120. The multi-ref interference suppressorreceives a downstream signal sample from each PA 132 via the respectiveTX coupler 136, and samples and converts this downstream signal to adigital form using a respective ADC 182. In the embodiment of FIG. 3,multi-ref interference suppressor 180 comprises four digital TD filters184, one TD filter per leg or ADC 182. Each TD filter comprises multipleTD coefficients configured so as to emulate the impulse response of theinterference channel between the relevant PA output and the RX referencepoint at the output of ADC 164.

Each of TD filters 184 applies a convolution operation between its(previously estimated) TD coefficients and digital samples of therespective sampled PA output, which contains, in addition to the DSsignal interfering components such as noise and distortion. An adder 186sums the outputs of the four TD filters to produce a combinedinterference suppression signal 188. A subtraction module 166 subtractscombined interference suppression signal 188 from the signal output byADC 164, to produce a partial interference-suppressed signal.

In alternative embodiments, the output of each TX coupler 136 isconverted to a digital signal and split into multiple reference channelsusing an US channelizer (e.g., such as US channelizer 168.) In suchembodiments, the multi-leg node comprises multiple interferencesuppressor modules 180 that each applies one channelization operationand multiple TD filtering operations to the multiple channels of therespective US channelizer. In these embodiments, subtraction module 166is located after US channelizer 168 of the Rx path.

As noted above, single-ref interference suppressor 190 receives a TXreference signal from a selected TX reference point in the TX chainpreceding DAC 120 (shown as a dotted line.) In the embodiment of FIG. 3,the TX reference point is configured before IFFT module 52, in whichcase single-ref interference suppressor 190 is implemented using a FDfilter. In an alternative embodiment, the TX reference point isconfigured after IFFT module 52, in which case single-ref interferencesuppressor 190 is implemented using a TD filter.

Single-ref interference suppressor 190 processes its input to produce aninterference suppression signal that a subtraction module 170 subtractsfrom the output of FFT module 172 to produce a finalinterference-suppressed signal at the input of FFT module 172. Whensingle-ref interference suppressor 190 is implemented using a TD filter,the subtraction point is after FFT module 172. Single-ref interferencesuppressor 190 may comprise one or more elements similar to the elementsof the digital part of the TX chain for compatibility between theselected TX and RX reference points. Single-ref interference suppressor190 further comprises a digital filter (not shown) that can beimplemented in the time domain or in the frequency domain as describedabove.

In some embodiments, the coupling output of each TX coupler 136 is splitinto multiple reference channels (as described above) and an FFToperation is applied per each of the reference channels of each leg. Insuch embodiments, the filtering operation of TD filter 184 is replacedwith a filtering operation in the frequency domain, and the subtractionpoint is configured after FFT module 172. In another embodiment, node100 combines both TD and FD filtering, which may result in a shorter TDfilter.

Design Considerations for a Multi-Leg Node

Next we describe various design considerations for the architecture ofmulti-leg node 100. The RX chain may experience various interferencetypes such as leakage and echoes of the transmitted signals. Otherinterference sources include thermal noise, quantization noise generatedby ADCs 164 and 182, and by DAC 120, and nonlinear distortion causedmainly by PAs 132. Considerations for mitigating the variousinterference sources will be described in detail below.

Due to limited isolation, the outputs of PAs 132 may leak through RXcouplers 148 into the RX chain and degrade the receiver performance. Inaddition, due to impedance mismatch, the downstream signal transmittedvia each of the node legs may be reflected as an echo from therespective RF connector and/or by taps and other elements in thedistribution network to which the leg connects. In general, the RX chainexperiences an interference that is the sum of the individualinterferences associated with the different legs.

Since PAs 132 transmit at a power level that is typically significantlyhigher than the typical receive level, the level of the interference ismuch higher than the received upstream signal, and therefore the dynamicrange of the RX chain, e.g., of LNAs 156 and ADC 164, should be designedsufficiently high to accommodate that signal to interference powerdifference.

In the architecture of FIG. 3, multi-ref interference suppressor 180accepts multiple TX reference signals from the outputs of respective PAs132. This configuration allows suppressing interference caused bythermal noise, quantization noise of DAC 120, and nonlinear distortioncaused by PAs 132. Note that such noise and nonlinear distortion effectsare invisible to single-ref interference suppressor 190 because its TXreference point is drawn prior to DAC 120.

Although the noise and distortion reflections are much weaker than thereflections of the TX signal, and are also typically weaker than theupstream signals received at the RX chain via the node legs, noise andnonlinear distortion may still limit the Signal to Noise Ratio (SNR)that can be achieved at the RX chain. Suppressing noise and distortionis therefore required for achieving high spectral efficiency.

In the example of FIG. 3, multi-ref interference suppressor 180 samplesthe outputs of PAs 132 and provides a combined suppression signal 188 tobe subtracted from the output of ADC 164, and therefore suppresses thedownstream signal including undesired components such as quantizationnoise of DAC 120 and distortion generated within PA 132.

For a given leg (of LEG1 . . . LEG4) the same transfer function modelsthe interference channel for the downstream signal output by therespective PA and the interference channel for the noise and distortionassociated with that PA. Note that although the same downstream signalis transmitted via all the legs LEG1 . . . LEG4, each leg experiencesdifferent noise and distortion interference corresponding to a differentinterference channel. Thus, the noise and distortion interferencechannels for the different legs are typically modeled by differentrespective transfer functions and therefore they cannot be modeled by acombined channel (as can be done for the downstream signal interferencechannel.)

The above discussion implies that for suppressing the noise anddistortion interference of the PAs, a dedicated suppression signalshould be generated for each leg. In the embodiment of FIG. 3, this isimplemented by using a separate TD filter 184 for each leg, as describedabove.

The level of noise and distortion interference is typically at least 30dB lower than the respective downstream signal, and therefore, areflected noise and distortion signal is received at a given leg atleast 30 dB lower than the respective reflected downstream signal atthat leg. This means that for achieving a certain interference residuallevel at the output of subtraction module 166, some of the TDcoefficients of TD filters 184 may be neglected (e.g., zeroed) comparedto the coefficients required for suppression of the downstream signal.

FIGS. 4A-4C are diagrams that schematically illustrate channel responsefunctions used for echo suppression, in accordance with an embodimentthat is described herein. The diagrams apply to an echo channel measuredover one of LEG1 . . . LEG4 of a typical CATV N+0 coaxial cable network.

FIG. 4A depicts the full channel response up to 6 microseconds. Theresponse value at time 0 corresponds to a reflection from the RFconnector of the leg. The subsequent peaks at about 0.4 microsecondsapart from one another correspond to six reflections from thedistribution network. In some embodiments, only part of the entirechannel response needs to be emulated for achieving a sufficient levelof interference suppression.

FIG. 4B depicts a subset of the channel response corresponding mainly toechoes of the downstream signal, and FIG. 4C depicts a subset of thechannel response corresponding mainly to distortion and noiseinterference. As seen in the diagrams, the number of nonzerocoefficients in FIG. 4B is much larger than in FIG. 4C. Therefore, afilter based on the coefficients of FIG. 4B may be implemented moreefficiently in the frequency domain, whereas a filter based on thecoefficients of FIG. 4C may be implemented more efficiently in the timedomain. Assume a total number Na of coefficients in FIG. 4A. Thecoefficients in FIG. 4B comprise a subset of the Nb most dominantcoefficients of FIG. 4A, wherein Nb<Na. The coefficients in FIG. 4Ccomprise a subset of the Nc most dominant coefficients in FIG. 4B,wherein Nc<Nb.

The nonzero coefficients in FIGS. 4B and 4C are arranged in clusters,wherein at least some of the clusters contain a small number of nonzerocoefficients. In an embodiment, only the nonzero coefficients areactually stored and multiplied by for filtering. Selecting the clusterscan be done using any suitable method. For example, the clusters may beselected so that each cluster contains a predefined number M ofconsecutive coefficients, while minimizing the power (or amplitude) ofthe coefficients that are zeroed.

In some embodiments, single-ref interference suppressor 190 applies FDfiltering, by applying FFT to the samples of the FD filter input,multiplying the FFT output by a frequency response of the interferencechannel derived, e.g., from the time domain coefficients of FIG. 4A, andapplying IFFT to recover the filtered time domain output to besubtracted at the input of FFT module 172. In another embodiment, asdepicted in FIG. 3, subtraction module 170 is positioned at the outputof FFT module 172, and subtraction module 170 performs the subtractionin the frequency domain.

In some embodiments, single-ref interference suppressor 190 applies FDfiltering, including conversion back to the time domain using IFFT, asdescribed above, by applying the FFT operation over partiallyoverlapping FFT blocks. The overlapping interval should be the same orlonger than the length of the interference channel response. Handling FDfiltering with overlapping FFT blocks can be implemented, for example,using the overlap-and-save method or the overlap-and-add method, asknown in the art.

In some embodiments, multi-leg node 100 transmits and receives OFDMsignals, e.g., in accordance with the FDX DOCSIS 3.1 standard citedabove. When the overlapping upstream OFDM signals received by the RXchain and the OFDM downstream signal of the TX chain are synchronized,e.g., as described in FIG. 2, FFT block overlapping for the FD filteringis not required for the downstream signal interference suppression. Notethat since noise and distortion interference may not have the structureof the OFDM signals, suppressing noise and distortion interference usingFD filtering (e.g., using multi-ref interference suppressor 180) withoutoverlapping may result in some residual interference.

The FDX DOCSIS 3.1 standard cited above supports a synchronizedoperation between downstream channels and overlapping upstream channels.For example, in DOCSIS full duplex, a DS channel may occupy thefrequency range between 108 MHz and 300 MHz, whereas one US channel mayoccupy a frequency range between 108 MHz and 204 MHz and another USchannel occupies the 204 MHz to 300 MHz range.

Based on the above described design considerations, the architecture ofFIG. 3 comprises a dedicated TD filter 184 for each leg, wherein each TDfilter comprising a small number of coefficients required forsuppressing noise and distortion interference for this leg using aconvolution operation. The TD filter may comprise coefficients forsuppressing also a downstream transmission that interferes with the RXchain via leakage and reflections. The TD filter may be applied for theentire full duplex band, or for a selected sub-band of the full duplexband, e.g., for a single OFDM channel. In some embodiments, the TDfilter is implemented in a clustered structure, storing only nonzerocoefficients of the TD filter, and calculating the convolution bymultiplying only the non-zero coefficients by respective input signalsamples. In alternative embodiments, the TD filter can be replaced withan equivalent FD filter, as described above. The FD filter may be fullyequivalent to the TD filter (in the case of using overlapping) orapproximately equivalent to the TD filter (when overlapping is notapplied).

The FD filtering operation of single-ref interference suppressor 190 isapplied for suppressing the residual interference that remains afterapplying the multi-leg interference suppressor. The FD filtering istypically applied separately per upstream OFDM or OFDM multiple Access(OFDMA) channel. Alternatively, the FD filtering may be applied to theentire full duplex bandwidth, or to any other suitable sub-band of thefull duplex channel, such as a downstream OFDM channel coinciding withone or more upstream OFDMA channels.

The configurations of transceiver 24, distribution network 40 andmulti-leg node 100 shown in FIGS. 1 and 3 are example configurations,which are chosen purely for the sake of conceptual clarity. Inalternative embodiments, any other suitable transceiver, distributionnetwork and multi-leg node configurations can also be used.

Each of the pairs of transmitter 28 and receiver 32, and/or TX chain 104and RX chain 108, may be integrated in a single device (e.g., on asingle silicon die) within transceiver 24 and within multi-leg node 100,respectively. Alternatively, each of transmitter 28, receiver 32, TXchain 104 and RX chain 108 is implemented in a separate device. Furtheralternatively, any other suitable partition of the elements comprised intransceiver 24 and multi-leg node 100 into separate devices can also beused.

The different elements of transceiver 24 and multi-leg node 100 may beimplemented using suitable hardware, such as in one or moreApplication-Specific Integrated Circuits (ASICs) or Field-ProgrammableGate Arrays (FPGAs).

In some embodiments, some elements of transceiver 24 and multi-leg node100, e.g., IFFT module 52 (and an IFFT module in TX chain 104 that isnot shown), FFT modules 76 and 172, digital interference suppressor 84,multi-ref interference suppressor 180 and single-ref interferencesuppressor 190 can be implemented using software, or using a combinationof hardware and software elements. Elements of digital transceiver 24and multi-leg node 100 that are not mandatory for understanding of thedisclosed techniques have been omitted from the figure for the sake ofclarity.

In some embodiments, some of the functions of transceiver 24 andmulti-leg node 100 may be implemented in a general-purpose processor,which is programmed in software to carry out the functions describedherein. The software may be downloaded to the processor in electronicform, over a network, for example, or it may, alternatively oradditionally, be provided and/or stored on non-transitory tangiblemedia, such as magnetic, optical, or electronic memory.

The embodiments described above are given by way of example, and othersuitable embodiments can also be used. For example, although thedescription above refers mainly to a transceiver in a headend of a cableTV system, the disclosed techniques apply similarly to other suitablecontent distribution systems and/or to a transceiver operating at thesubscriber side.

Although the embodiments described herein mainly address interferencesuppression in communication systems based on the DOCSIS 3.1 full duplexspecifications, the methods and systems described herein can also beused in other applications, such as in applying other communicationtechnologies, e.g., Digital Subscriber Line (xDSL), Ethernet, Satellitecommunication, and 5G mobile systems.

It will be appreciated that the embodiments described above are cited byway of example, and that the following claims are not limited to whathas been particularly shown and described hereinabove. Rather, the scopeincludes both combinations and sub-combinations of the various featuresdescribed hereinabove, as well as variations and modifications thereofwhich would occur to persons skilled in the art upon reading theforegoing description and which are not disclosed in the prior art.Documents incorporated by reference in the present patent applicationare to be considered an integral part of the application except that tothe extent any terms are defined in these incorporated documents in amanner that conflicts with the definitions made explicitly or implicitlyin the present specification, only the definitions in the presentspecification should be considered.

1. An apparatus for communication, comprising: a transmission chain,configured to transmit a transmitted signal; an interference suppressor,configured to produce one or more suppression signals, by filtering oneor more respective reference signals drawn from the transmission chain,wherein at least a given reference signal of the one or more referencesignals is filtered in a frequency domain; and a reception chainconfigured to: receive a received signal distorted by an interferencesignal comprising at least an attenuated and delayed replica of thetransmitted signal, wherein the transmitted signal and the receivedsignal are (i) synchronized in time and (ii) each comprises one or moresubcarriers selected from a common subcarrier-resource; process thereceived signal up to one or more selected points along the receptionchain, and respectively subtract the one or more suppression signalsfrom the received signal at the selected points to produce aninterference-suppressed signal; and recover data carried in the receivedsignal from the interference-suppressed signal.
 2. The apparatusaccording to claim 1, wherein the transmission chain comprises a digitalTX sub-chain followed by an analog TX sub-chain, wherein each of the oneor more reference signals is drawn from the digital TX sub-chain or fromthe analog TX sub-chain.
 3. The apparatus according to claim 1, whereinthe reception chain comprises an analog RX sub-chain followed by adigital RX sub-chain, wherein each of the selected points along thereception chain is comprised in the analog RX sub-chain or in thedigital RX sub-chain.
 4. The apparatus according to claim 1, wherein thetransmission chain and the reception chain are configured to operate ina full duplex mode.
 5. The apparatus according to claim 1, wherein theinterference suppressor is configured to generate an analog suppressionsignal based on a digital reference signal drawn from a digitalsub-chain of the transmission chain.
 6. The apparatus according to claim1, wherein the interference suppressor is configured to generate, usingtime-domain filtering, an analog suppression signal for suppressing atleast noise or distortion interference, and to generate, usingfrequency-domain filtering, a digital suppression signal for suppressinginterference caused by leakage or reflections of the transmitted signal.7. The apparatus according to claim 1, wherein the transmission chain isconfigured to split the transmitted signal into multiple signal-copiesfor transmission over multiple respective ports, wherein the receptionchain is configured to combine multiple reception signals received fromthe respective ports, and wherein the interference suppressor isconfigured to generate a suppression signal based on respective samplesof the multiple signal-copies.
 8. The apparatus according to claim 7,wherein the interference suppressor is configured to generate for themultiple signal-copies multiple respective temporary suppressionsignals, and to generate the suppression signal by summing the multipletemporary suppression signals.
 9. The apparatus according to claim 8,wherein the interference suppressor is configured to generate thetemporary suppression signals by applying to each of the signal-copies atime-domain or a frequency-domain filtering operation.
 10. The apparatusaccording to claim 8, wherein the interference suppressor is configuredto generate the suppression signal for reducing a level of at leastnoise or distortion present at the multiple signal-copies.
 11. Theapparatus according to claim 1, wherein the interference suppressor isconfigured to subtract a first suppression signal from the receivedsignal at a first point of the reception chain, and to subtract a secondsuppression signal at a second different point of the reception chainfor reducing a residual interference signal present in the receivedsignal after subtracting the first suppression signal at the firstpoint.
 12. The apparatus according to claim 1, wherein the transmissionchain and the reception chain are configured to operate in accordancewith a DOCSIS full duplex specification.
 13. The apparatus according toclaim 1, wherein the transmission chain is configured to transmit thetransmitted signal over a coaxial-based network of a Cable TV (CATV)system, and the reception chain is configured to receive the receivedsignal over the coaxial-based network of a Cable TV (CATV) system.
 14. Amethod for communication, comprising: transmitting a transmitted signalby a transmission chain; producing one or more suppression signals, byfiltering, one or more reference signals drawn from the transmissionchain, wherein at least a given reference signal of the one or morereference signals is filtered in a frequency domain; receiving in areception chain a received signal distorted by an interference signalcomprising at least an attenuated and delayed replica of the transmittedsignal, wherein the transmitted signal and the received signal are (i)synchronized in time and (ii) each comprises one or more subcarriersselected from a common subcarrier-resource; processing the receivedsignal up to one or more selected points along the reception chain, andrespectively subtracting the one or more suppression signals from thereceived signal at the selected points to produce aninterference-suppressed signal; and recovering data carried in thereceived signal from the interference-suppressed signal.
 15. The methodaccording to claim 14, wherein the transmission chain comprises adigital TX sub-chain followed by an analog TX sub-chain, wherein each ofthe one or more reference signals is drawn from the digital TX sub-chainor from the analog TX sub-chain.
 16. The method according to claim 14,wherein the reception chain comprises an analog RX sub-chain followed bya digital RX sub-chain, wherein each of the selected points along thereception chain is comprised in the analog RX sub-chain or in thedigital RX sub-chain.
 17. The method according to claim 14, wherein thetransmission chain and the reception chain operate in a full duplexmode.
 18. The method according to claim 14, wherein producing the one ormore suppression signals comprises generating an analog suppressionsignal based on a digital reference signal drawn from a digitalsub-chain of the transmission chain.
 19. The method according to claim14, wherein producing the one or more suppression signals comprisesgenerating, using time-domain filtering, an analog suppression signalfor suppressing at least noise or distortion interference, andgenerating, using frequency-domain filtering, a digital suppressionsignal for suppressing interference caused by leakage or reflections ofthe transmitted signal.
 20. The method according to claim 14, andcomprising splitting the transmitted signal, by the transmission chain,into multiple signal-copies for transmission over multiple respectiveports, combining, by the reception chain, multiple reception signalsreceived from the respective ports, and generating a suppression signal,by the interference suppressor, based on respective samples of themultiple signal-copies.
 21. The method according to claim 20, andcomprising generating for the multiple signal-copies multiple respectivetemporary suppression signals, and wherein generating the suppressionsignal comprises summing the multiple temporary suppression signals. 22.The method according to claim 21, wherein generating the temporarysuppression signals comprises applying to each of the signal-copies atime-domain or a frequency-domain filtering operation.
 23. The methodaccording to claim 21, wherein generating the suppression signalcomprises generating the suppression signal for reducing a level of atleast noise or distortion present at the multiple signal-copies.
 24. Themethod according to claim 14, wherein subtracting the one or moresuppression signals comprises subtracting a first suppression signalfrom the received signal at a first point of the reception chain, andsubtracting a second suppression signal at a second different point ofthe reception chain for reducing a residual interference signal presentin the received signal after subtracting the first suppression signal atthe first point.
 25. The method according to claim 14, wherein thetransmission chain and the reception chain operate in accordance with aDOCSIS full duplex specification.
 26. The method according to claim 14,wherein the transmitting the transmitted signal comprises transmittingthe transmitted signal over a coaxial-based network of a Cable TV (CATV)system, and wherein receiving the received signal comprises receivingthe received signal over the coaxial-based network of the CATV.